1. Field of the Invention
The present invention relates to an exhaust emission control system for an internal combustion engine which supplies secondary air into an exhaust passage to oxidize and remove HC and CO in an exhaust gas, and which introduces an exhaust gas into a cylinder in an intake stroke to prevent generation of NOx during combustion.
2. Description of the Related Art
Japanese Utility Model Registration No. 2535339 discloses an exhaust emission control system for an internal combustion engine including: a secondary air passage connected to an exhaust passage; and a one-way valve that is provided in the secondary air passage and opened by negative pressure by exhaust pulsation in an exhaust stroke in order to supply secondary air into the exhaust passage to oxidize and remove HC and CO in an exhaust gas. Japanese Patent Application Laid-Open No. 2005-240793 also discloses an exhaust emission control system for an internal combustion engine including an exhaust gas recirculation device that operates so as to partially open an exhaust valve to introduce an exhaust gas into a cylinder in an intake stroke in order to prevent generation of NOx during combustion by introducing exhaust gas into the cylinder in the intake stroke.
For an internal combustion engine having high muffler back pressure or a short exhaust passage length, sufficient negative pressure by exhaust pulsation is not created in an exhaust passage. Thus, even if the exhaust emission control system disclosed in the above-described Japanese Utility Model Registration No. 2535339 is employed, the one-way valve in the secondary air passage is not opened, and secondary air cannot be supplied to the exhaust passage, thereby preventing a desired effect from being obtained. In such a case, an air pump is connected to the secondary air passage to pump the secondary air into the exhaust passage, but the use of the air pump increases the cost of the exhaust emission control system.